Alginates are isolated from marine brown algae. Alginate is also produced in soil bacteria such as Azotobacter vinelandii and Azotobacter crococcum and several different Pseudomonas sp. Brown algae are however generally the source of commercially available alginates.
Alginates are used in foodstuffs and in pharmaceutical, dental, cosmetic and other industrial products. The most common industrial applications are based on their hydro colloidal and poly electrolytical nature, which forms the basis for the gel-forming, thickening, stabilizing, swelling and viscosity-providing properties.
Alginates are salts of alginic acid, a linear, hetero polysaccharide consisting of (1-4) linked β-D-mannuronic acid, designated as M, and α-L-guluronic acid, designated as G. These two uronic acids have the following formulae:

The polymers exist as homopolymer sequences of mannuronic acid, called M-blocks, homopolymer sequences of guluronic acid called G-blocks, and mixed sequences of mannuronic and guluronic acid units, designated MG-blocks or alternating blocks.
The following scheme represents an illustration of the structure of alginates:
MMMMMMMGGGGGGGGMGMGMGMGMGGGGGGGGM  M-block G-block MG-block G-block
Alginates usually contain all three types of blocks and a block typically consists of three to thirty monomer units. The distribution of the blocks depends on the type of algae from which the alginate is isolated, as well as on the age and part of the plant. For example alginate from the stalk may have a different sequence and block composition compared with alginate isolated from the leaves. The time of year at which the algae are harvested also affects the block composition and sequence. According to common knowledge, the highest G-content can be found in the stem of old L. hyperborea. The leaf of the same species has a somewhat lower G-content and shorter G-blocks, but the content is still higher than most other species. Commercially available alginates usually have a G-content of 25%-70%.
Sources which have a high content of M-blocks are for example species of the brown algae genera Durvillea, Laminaria, Lessonia, Fucales and Ascophyllum, in particular the fruiting bodies of Ascophyllum during spring are rich in mannuronic acid.
Alginates which are rich in mannuronic acid have been shown to possess immunostimulating activity as described in U.S. Pat. No. 5,169,840.